When short-circuiting occurs in power distribution systems using conventional fuses very large short-circuit currents occur, these are serious transient over- and undervoltages as a result. In telephone stations, for example, undervoltages cause the cessation in the functioning in the electronics cease with the risk of long operational interruptions. Overvoltages carry the risk of destroying electronic circuits. It is already known, e.g. from Ericsson Review No. 4, 1974 page 120, to solve the problems occurring in short-circuiting by making the power system "high-ohmic". The appended FIG. 1 illustrates an example of such a high-ohmic system. The rectifier RL supplies a load L via a filter F, a fuse S1 and a cable K. A battery B with a voltage E emk and internal resistance R.sub.i is connected as a standby to the system. The fuses S2-S6 are connected to further, unillustrated loads. The high-ohmic situation in the system means that the resistances in the various circuits are distributed between battery and cable in the ratio of 1:10, for example. This means that the internal impedance R.sub.i of the battery and the line impedance of the cable K do not fall below certain values. For a 48 V system, for example, R.sub.i =4.5 m .OMEGA. and the cable resistance R.sub.K =45 m .OMEGA. in spite of this not being desirable from the point of view of losses. Due to the high-ohmic situation, the short-circuiting current occurring is therefore limited in this example to a maximum of 1000 A. This solution gives a voltage drop of max 4.5 V. in the distribution, even for a short circuit. Loads connected to fuses, i.e. to S2-S6, other than the short-circuiting load L can therefore be kept at an acceptable voltage level.
For a short circuit in the load L there is obtained a transient sequence at the point A (FIG. 1) illustrated in the accompanying FIG. 2. FIG. 2 shows in a diagram that the voltage falls rapidly at the short-circuiting instant, and is thereafter constant, to rise rapidly at the instant when the fuse S1 has melted. With a high-ohmic system according to the above (graph b) there is obtained a maximum voltage drop and a maximum voltage rise of about 4.5 V, respectively, at the short circuiting instant and when the fuse has melted. The curve plotted with a full line illustrates the sequence for a conventional relay system which is not high-ohmic, and the chain-dotted curve c the sequence when using an electronic current interrupter in accordance with the invention. The high-ohmic solution to the transient problem described above requires large batteries, however, and is therefore only possible in practice for large power distribution systems. In small systems with batteries of less than about 2000 Ah, very large capacitors are required to keep the transients between acceptable levels.